1. Field of the Invention
The present invention relates to a lift safety for a venetian blind. More specifically, the invention relates to a lift safety having a carriage which is coupled for moving upward and downward with at least one of the slate of the blind. The carriage moves linearly along stationary frame parts of the blind, and has at least one stop-pawl lever on it. The lever swings about a swing point in relation to the carriage within the plane in which the blind lifts. The end of the stop-pawl lever comes into frictional contact with one of the frame parts. The lever is clamped against that frame part by linear displacement of the swing point upon the raising of the carriage.
The invention also relates to a drive transmission for the lowering and raising of a venetian blnd, an anti-twist safety for slats of venetian blinds and a venetian blind having at least one of these devices. More specifically, the invention relates to a drive transmission for the lowering and raising of a venetian blind by means of a first strap-like element and for the tilting of the slats by means of a second strap-like element. The first element travels on a drive member while the second is mounted on a driver arrangement which is mounted for rotary movement with respect to the drive member.
2. Description of the Prior Art
German Application DE OS No. 30 37 733 discloses one such lift safety to prevent unauthorized raising of the slats of a venetian blind. A locking lever is provided on a carriage which is longitudinally displaceable in guides. The lever's length is greater than the distance between its point of swing on the carriage and the portion of the carriage guides against which the lever engages. Upon unauthorized raising of the carriage i.e. upon raising the blind without the drive device provided for raising or lowering it, the lever engages the corresponding portion of the carriage guide and locks against the guide. Once locked, the lever remains in a position in which it is inclined obliquely with respect to the raising direction.
This known lift safety has the following disadvantages: Due to the fact that the lever locks in a position inclined at an oblique angle with respect to the raising direction of the blind, the safety may gradually give way due to vibrations of the carriage. The lever cannot be dimensioned in such a manner that it is eventually pressed by continued raising of the carriage into a more stable perpendicular position with respect to the raising direction because the lever length would then have to be fitted very precisely to the distance between its point of swing on the carriage and the lock-guide portion in order that the pawl would swing into the locking position. On the other hand, large tolerances with respect to the dimensions and the nature of the surface of the carriage guides must be taken into account. These tolerances make designing of a lever as mentioned above problematical.
A drive arrangement is also disclosed in the above-mentioned German Application DE OS No. 30 37 733. A drive wheel of a lifting tape is driven by a shaft for lowering or raising the blind. The drive wheel is connected to a driver device through a coil spring arranged on the circumference of the shaft. The driver device comprises a bushing which is coaxial to the shaft and is seated over the coil spring. The coil spring acts as a frictional coupling between the drive wheel and the drive device and the drive arrangement as long as the ends of the coil spring extending through the bushing are not in contact with stops. The desired stops are arranged in the paths of the spring ends, which act as counterstops, so that the torsion spring comes in contact with one of the stops upon the raising or lowering of the blind. In positions of the blind in which the inclination of the slats is to be changed, however, the spring is released. This permits the drive arrangement to turn with the drive wheel.
This arrangement has the disadvantage of a relatively complicated construction. Also, upon normal raising and lowering of the blind, friction must be overcome. With the torsion spring stopped and fixed against rotation, this friction is produced between the drive wheel's shaft and the spring. As a result, the drive unit must be designed with due consideration of this friction. The friction must not be less than a certain amount, because this friction is necessary for the rotary connection between the drive wheel and the drive device.